Volunteers will be randomly assigned to sleep overnight in the Metabolic Clinical Research Unit at NIH, set at either mild cold (19degree C/66degree F) or mild warm (27degree C/81degree F), with a lead-in period and a wash-out period before crossing over, during which the temperature will be ambient (24degree C/75 degree F). Volunteers can conduct their regular activities during the day. Each period will be 1 month in duration. At the end of each month, volunteers will be evaluated for their metabolic response to a mild cold challenge for 24 hours (19degree C/66degree F) to determine whether brown fat function and activity have been altered by temperature acclimatization in the previous month.